In the last few episodes, I’ve spoken a great deal about what we know about
the birth of the universe, beginning with the traditional theory of the Big Bang and
working backward to what we know about the actual moment of creation itself.
But the other end of the tale of the cosmos is equally interesting – how
will it end? That’s the story I want to tell in this week’s episode of Subatomic Stories.
Humanity has long been fascinated by questions of the end of time,
with the two most common predictions being that the universe would end in a cataclysm of fire,
or a frigid eternity of ice. Indeed, literature abounds with such tales,
beginning with the very first writing. So, the question is old, but like all such big questions,
science has uncovered a pretty good handle on what is going to happen. So, let’s talk about it.
The story begins, well, at the beginning. The Big Bang happened. The universe is
currently expanding. These are facts of today. The question is what comes next?
Well, expanding means moving apart, so, in the absence of other things, the universe
will expand forever. But, of course, other things are relevant. Gravity pulls things back together.
And thus, the question becomes whether gravity or expansion will win. Will the universe expand
forever, ever cooling? Or will gravity win, causing the motion of matter to reverse,
dragging matter back inward, heating itself hotter and hotter, in what is called a Big Crunch?
You see, the age-old question of fire or ice is a scientifically valid one.
With the observation in 1998 of the accelerating expansion of the universe,
we now know the answer. Dark energy will win,
and the universe will expand and cool. That’s it. The end. This was a short video.
Well, there are more things to talk about. Let’s think about the things
we know and then talk about some things that are still not entirely figured out.
Dark energy, which I talked about in episode 23, is a constant energy density.
That means, as the universe expands – which is to say increases in volume – the amount
of dark energy increases. Since dark energy pushes the universe apart, more dark energy
means faster expansion. What consequence does that have for the future of the universe?
We know the answer to this, and it’s kind of cool when you hang some real numbers on it.
It boils down to working out how much mass exists in any particular volume,
compared to the amount of dark energy in the same volume. You can do the calculation
yourself if you want. Start with the dark energy density. Converting energy to matter,
dark energy works out to be about seven times ten to the minus thirty grams per cubic centimeter.
For the Earth, the amount of matter is about a quadrillion,
quadrillion times more than the dark energy – that’s ten to the thirty power times more.
For the solar system, you can add up the dark energy in a sphere with the radius
of the orbit of Neptune and compare it to the mass of the solar system. There’s about
a hundred quadrillion times more matter than dark energy, or ten to the seventeenth power.
And you can even do it for galaxies. For instance, the dark matter surrounding the Milky Way extends
in a sphere about a million lightyears in radius. So, you can take that huge volume,
calculate the dark energy, and compare it to the amount of matter in the Milky Way’s spiral
and the dark matter cloud surrounding it. You find that the amount of matter in the
vicinity of the Milky Way is about 200 times more than the dark energy.
So, these numbers tell you something. It means that in the Earth, the solar system,
and even in the vicinity of the Milk Way, there is a lot more matter than dark energy.
So that means that the attractive form of gravity wins,
and dark energy isn’t strong enough to cause any of these things to expand.
Now when you get into much bigger sizes,
where there are nearly-limitless tracts of intergalactic space,
all of which is nearly empty of matter, then dark energy eventually wins and becomes larger per
volume than the density of ordinary matter. And, at these large scales, dark energy begins to win.
So, for the future of the universe, that means that with objects like galaxies that are clumped
together, they stay together. In our case, while the expansion of the universe will carry distant
galaxies away from us, eventually so fast that they will fade away into invisibility,
what is called out Local Group of galaxies are near enough that
they will stay with us due to their strong gravitational attraction. The Local Group,
in case you don’t know, consists of galaxies in a sphere about ten million lightyears across.
It contains the Milky Way and Andromeda, and then maybe eighty additional small galaxies.
So, for the future. Currently Andromeda and the Milky Way are on a collision course.
They will collide in about 4.5 billion years, eventually turning into a big elliptical galaxy
that astronomers call Milkomeda. Eventually, the other dwarf galaxies will join. With the more
distant galaxies carried away by cosmic expansion, astronomers in the far future will see a universe
as containing a single large elliptical galaxy, surrounded by absolutely nothing.
And, of course, the stars will burn during all of that time. About the time Milkomeda is formed,
the sun will have used up its hydrogen fuel and expanded into a red giant. It will stay
in that state for a billion years or so, and then collapse, eventually forming a white dwarf, which
is a star in which fusion has stopped. It will glow faintly for hundreds of billions of years.
And that story will happen over and over again, eventually with all stars burning out.
Some will collide and form black holes, which will then slowly evaporate over the eons
via what is called Hawking radiation. As the stars wink out, one by one,
the universe will slowly die a cold and lonely death – not with a bang,
but with a whimper, as poet T.S. Elliott once penned. Very sad.
So, is that the fate of the universe? Well, it well may be. Certainly, that’s what it appears
like the future of the universe will be. But, of course, we’re now talking hundreds
of billions of years in the future. The laws of physics might change. If the idea of eternal
inflation that I mentioned in the last episode is real, then maybe a new cycle might happen.
Indeed, there is a lot of room for all sorts of unexpected things to arise. What I’ve described
here is our current best guess, but we should be open to new discoveries, which may point us in a
different direction. The answer to the question of the end of the universe has not yet been
definitively told. And, speaking of questions, what sorts of questions do you have for me today?
Question time is always interesting. Let’s see what you got.
Change Gamer asks why distant galaxies move away faster than nearby ones. Hi Change. I think…and I
admit I’m guessing here…that you might be thinking about the situation incorrectly. Galaxies aren’t
moving away from us, at least not in the general sense of the meaning of the word “moving.”
Rather, what is happening is that galaxies are generally stationary, but space is stretching.
Imagine three galaxies, A, B, and C. They are all sitting there, stationary compared to the
space in which they sit. You can think of them as three dots painted on a stretchy bit of rubber. Or
you can think of them as galaxies. Whatever works for you.
The three galaxies are located in a straight line and an equal distance apart. Initially,
the distance between A and B is little d. The distance between A and C is twice that, or 2-D.
Now let’s stretch space and see what B and C do, if we take the point of view that
we are in the same location as A. Thus, from our point of view, A is stationary.
Now let’s double all distances, which happens in a time that we’ll call T.
If we double the distances, the A-B distance goes to 2D and the AC distance goes to 4D.
Velocities are figured out by taking the change in distance and dividing by time. So the velocity
of B is 2D minus D, all over T. That’s D over T. The velocity of C is 4D minus 2D over T. That’s 2D
over T, or twice the velocity that A has. The bottom line is that it’s all about stretching
space, not actual motion. And more distant locations seem to move more. Good question.
Several viewers are having a hard time getting their head around what scientists mean by flat
space. It isn’t helped by the images people often see in videos. For instance, we often show flat
space as a two dimensional grid. And that makes people think about flat like a table top. We also
compound the issue when we show a two dimensional curved space. It’s all a bit confusing.
Now, it’s completely understandable why people think that flat space means two dimensional,
but that’s not what scientists mean. We mean a flat three dimensional space.
This is what I mean by a flat, three-dimensional, space.
It’s a simple and regular grid. Now if I bend that 3D space, like happens in general relativity,
you see that the grid is distorted. This space is no longer flat. And, if you’re interested
in how moving matter can distort space, here’s an animated gif that shows that too.
So, why do scientists use flat 2D space to illustrate what we mean? Basically it’s
just simpler to do, and, once you get your head around the fact that it’s just a slice through
a 3D space, it can help you understand. But it can be confusing. If you want to have a better
understanding of the dimensionality of space, I recommend the book “Flatland” by Edwin Abbott.
It’s well over a century old, but it’s worth your time. Be prepared to have your mind blown.
Decoding the Universe asks where the Higgs boson gets its mass if the Higgs boson gives mass to
other particles. Hi Decoding. Actually, I’m sorry to say, the Higgs boson doesn’t give mass
to particles. The Higgs Field does. Now, I can’t describe in detail the differences here, but I’ve
put two URLs for two videos in the description that go over that topic. They’re worth your time.
Very briefly, you can think of the Higgs field as water, and the Higgs boson as a water molecule.
It’s an imperfect analogy, but it helps in this instance. Objects moving through water interact
with all the water, not individual molecules. In this analogy, the Higgs field giving mass to
Higgs bosons, is a bit like viscosity, where individual water molecules interact with all
of the molecules around them. The analogy is a bit sketchy, but it mostly works for me.
Bobbias asks how we know that inflation occurred so early compared to recombination. Hi Bobbias.
Finally a question that’s easy to answer. It’s because of the uniformity of the cosmic microwave
background radiation. If inflation happened later in the process, then there would have
been some time for matter and energy to clump, and then the inflation would have enlarged the
non-uniformities. Since the CMB is so uniform, inflation must have been super early in the
process, before non-uniformities could have arisen. It’s that easy.
Jenaf notes that saying “the universe expanded at the speed of light”
is a sloppy on. Hi Jenaf. You’re absolutely right. This is very confusing. There is no
one expansion speed. However, if the universe is expanding, the statement is still correct.
In fact, it’s true today. For instance, we know from measurements that the current expansion of
the universe is the Hubble constant, which is 70 kilometers per second per megaparsec of distance.
A megaparsec is a million parsecs, or 3.26 million light years, but the details don’t matter.
Big picture, a megaparsec is just a characteristic distance. And the Hubble constant says that
objects a megaparsec away are moving away from us at 70 kilometers per second. Two megaparsecs,
140 km/second, three megaparsecs, 210 km/sec, and so on. And, eventually,
at a distance of 4285 megaparsecs, those objects are moving away from us at the speed of light.
So that’s how it works. Objects closer than that distance move slower than light.
Objects at that distance move at the speed of light. And objects more distant move faster
than light. That’s all there is to it. And the distances and speeds differ over time,
but the big idea is the same. There is a distance beyond which objects move faster than light,
and that’s what we mean when we say that universe expands at the speed of light.
Yashesh Ratnakar asks what caused inflation. Hi Yashesh. The simple answer is we don’t know.
The question is what gave the universe the energy to expand? There are some ideas.
One idea is what is called a phase transition. A phase transition is like when water freezes.
When water freezes, an enormous amount of energy is released,
even though the temperature doesn’t change while it freezes. Enormous.
So, one idea is that early in the history of the universe, all the known forces were the same and,
as the universe expanded and cooled, the known forces differentiated themselves, like electricity
and magnetism appear different, even though they are the same thing. Some scientists think
it is possible that when the strong nuclear force split off from the other forces,
this phase transition provided the energy to drive inflation. This idea is not universally accepted,
but it gives you a sense of where the energy that drove inflation might have come from.
Okay, so that’s all the time we have for questions today. If you enjoyed the video, please like,
subscribe, and share. Tell all your friends about the amazing physics you learned about here.
They may enjoy it themselves when they’re sitting around at home because, well, you know,
even at home, physics is everything.